R.I.P.
I have always loved the Beldum family and one of my best PoGo moments was to find a 100% beldum in the wild. Eventually I had enough candy to evolve it all the way and I did.
If I knew then what I know now I would never had evolved my Beldum. So Niantic, Seriously, why cant we TM to the special move during the 3 hour window?
It will hold back all my next "big mon evolves". I will most likely not longer just evolve them but wait for a CD.

Answers
No offense... but why did you evolve it two weeks ago? They introduced the BS no-TM rule with the second community day (Dratini), and the third community day (Bulbasaur) showed that there could be exclusive moves that dramatically improve a pokemon.
I got a pretty good Beldum and Bagon early on, but waited because they had crap charge moves and I saw no current need to rush either of them. When they started no-TM exclusive moves, that was all the more reason to wait.
It is a stupid rule they have imposed, but that is the game we now have to play.
Niantic don't allow TM's for the special move during the three hour window because it would reduce the value of the special move. Thus people have Less incentive to do community day which means less money for Niantic.
I didn't know you could get the special move by evolving a previously caught Mon. I tried that on a Dratini during the community day and it didn't work.
"Waste"? Come on.
First, how far are we from Beldum CD? Best case scenario one month, more realistically 3 months or maybe much much longer. But that's not the important part.
Metagross has basically 2 niches: bulky psychic attacker that resists steel attacks (in your face HS Machamp) and steel attacker. Very likely, the CD will give it a move that improves its performance in *one* of those roles, or create another useful niche. But it cannot make *all* variants of Metagross obsolete. So, my suggestion would be: during CD, evolve your bests Beldums to get the new attack and make them champions on that role. Then, TM this previously evolved 100% so that it takes the other role (meaning: if the CD gives him a better psychic attack, TM it to double steel ; if the CD gives it a steel move, keep it as is). Your 100% Metagross will still be the best possible in that role.
It would have sucked for Venusaur, which has only one relevant role, but Metagross has the chance of having several. I don't think many people complained about their 100% B/C Tyranitars after announcement of Larvitar CD right?
Conclusion: congrats for this great monster, I bet it will serve you well ;)
Thank you, finally some one who sees the value of previously evolved CD pokemon that cand get their event moves. YEs the starters got screwed over if they dont get their event moves but byt they are still kept even with nonevent pokemon of the same type and will all be outclassed by something eventually, as well as with these events, its likely niantic's going to stock us up on starters prior to each event, but all the other event pokemon that are already given their moves have a secondary use, ampharos' electric moves, Tyranitar's dark set, eevee's evolutions STAB sets, (dragonite's an oddity because draco meteor is slightly worse than outrage and draco meteor was its event move).
I hope we dont get another tyrantiar situation where everyone forgets about a pokemon's secondary type set after its event move and think that "X pokemon cant get event move, must be trash" (yes, I have seriously heard people saying if a tyrnatiar cant get SD its trash even if they dont have any dark tyrnaitar)
I think a lot of us are in that same boat; I've got about 20 Pokémon that I'm not evolving right now (a 100% Totodile, a bunch of 90%+ 2nd/3rd Gen Starter Mon's, ALL of my Bagons (I only have 1 good one so far anyway), a 98% Vigoroth (Wild Caught Level 30!) and at least 5 Ralts/Kirlia. They're sitting in my inventory, waiting for their respective Community Day's to be Evolved (or in some cases, traded/discarded). After the Mareep Day where it became a pattern that Community Day Pokémon would get an exclusive move, I stopped planning to evolve any Pokémon which could get a CD in the future.
Sadly, my first hatched Beldum was a 96%, and I was so excited about it that I immediately evolved it to a Metagross before remembering that it could get a Community Day. So I've got a Psychic Metagross that I use exclusively as a Gym Defender right now (mostly to let other players get the occasionally Beldum candy than anything else).
Mareep day didn't really cause any worries yet for me because the exclusive move was completely meta-irrelevant. It was FP Venusaur that made me worry and stop evolving.
I also have a 96% Metagross but I have no regrets. He has about 1k wins from all the times I brought him in to demolish fairy-types and now he'll live on as one of my better psychic options for unique solos.
The solution that would make everyone happy (including Niantic) is this:
If you evolve to the special move during a cd, and then transfer the mon to the professor, the professor pops up and asks you "You are transferring a pokemon that knows a very special move, would you like to teach that move to another pokemon of the same type".
Done - you still have to catch and evolve during CD, but you can transfer the move to an already powered or better IV pokemon.
I like the idea of awarding an HM (hidden machine) as a reward for catching some high number (50-100) of the CD species. The HM could then be used to teach an exclusive move to any mon that has one, including CD alumni, the Gen 1 birds, Snorlax, & Mewtwo (after tomorrow). This would encourage CD participation, even if the move or mon is lackluster.