To Level 40 your pokemon or Not to level 40 your pokemon that is the question.
I don't mean this as a debate on whether this is breaking the rules or not breaking the rules.
39 Cap might be a bug, and 3 phone/airplane mode is just a workaround for a bug.
39 Might be meant to be a purposeful cap, and the 3 phone/airplane mode is not right.
No one other than Ninatic can definitively answer that, so that is NOT what I am asking.
I am asking what YOU personally are going to do.
Answers
I was having this dilemma a few weeks ago. I ended up taking two of my level 37.5 Pokemon to level 39. I liked the idea of the power up button not being visible anymore. I'm really on the fence with some of my 100% pokemon though. It would be nice to have an absolutely perfect level 40 Pokemon, the best you can have but at the same time I don't know what Niantic truly intended. It makes more sense to power pokemon to 40 than level 39, especially with the max level currently being level 40. I'm probably going to end up powering all my non perfect pokemon go level 39 and then maybe think more about powering the 100% ones to 40. There is really no right or wrong answer to this question though.
I honestly think it is a bug and Niantic just doesn't know about. It just seems way too weird for the trainer level cap to be 40 and have the Pokemon level cap only be 39 even though they are coded to be able to go to 40. This is just a guess but, I think that Niantic does not expect players to realize that the Pokemon have levels too and instead they just expect players to look at the level arc instead. Basically Niantic probably doesn't expect their players to know so much about their game
Luckily for me however I won't have to worry about this for a long time since I am only level 30 after playing for a year and a few months.
If the multi-finger trick no longer works, then Niantic must be aware of it. If a method to bypass the lvl 39 cap and get your Pokémon to lvl 40 instead was patched, then Niantic is clearly capable of actually fixing this bug so that you can power up your Pokémon the natural way. Why Niantic has yet to bring this to attention is beyond me, though.
I am personally going to start maxing out my Pokémon to lvl 40 now. I already have over 30 Pokémon maxed at the lvl 39 cap, but it still feels incomplete to me. I don't really see why that would be an issue, the max level is clearly 40, not 39. It's coded in the game, and should not be considered a broken mechanic. If I can still power up my Pokémon to lvl 40, then it should be fine to do so.
Level 40 might be in the code, but you cannot get there by normal in-game means, so trainers who don't consult forums (I am sure there is lots of those) just won't get there. So I would consider it an unfair advantage in a PvP scenario for sure.
These sophisticated debates about whose fault it is that you can only get there with a work-around and what you are consequently entitled to do as a shrewd player are completely missing the bigger picture imo.
I doubt by the time that players are 38/39/40 they "Don't visit forums" or at the very least havent heard about this at raids. People at our raids have talked about it.
That's like saying I'm sure there are people who don't use IV Calculators and just use in game estimate. IV Calculators aren't in the game, so the fact that you know where to mid-max dust expenditure is unfair in PVP.
