For me personally in the games I DM, any WOTC character build choice is available. Period. I see that as part of player agency. If players want to "ruin the game" with OP options being used outside of how they are intended or in combinations that the designers did not forsee that is their purview. Some may say it breaks the story, but the PC character build is mostly the player's story, not mine. So as long as it is within the rules, I won't get in the way of that. If other players have an issue, that is kind of on the players to solve between them or in the social contract before session 1. IME it is rarely/never a problem for other players.
When you consider things like Dragonlance or Strixhaven backgrounds, those are explicitly OP options compared to other backgrounds, yet I have never banned them outside of those settings. I have also never had a player want to choose one of them outside of Dragonlance (I have never played Strixhaven).
As far as being a problem for me as DM. I have the ultimate trump card. I can scale encounters and I can challenge any possible build, making new monsters from scratch if necessary. Players can build an OP character, but they can not build a character too OP for a DM to challenge.
Rules that are not part of character design or character build are typically discussed in session 0. Things like tumble (usually allowed), flanking (never allowed), climb on monster (always allowed), marking (typically not allowed) .....
Usually, I know what I like best and works best for games I have played before, but usually I am up for a discussion if someone really wants or does not want something. If I was faced with the "martial issues" many on this thread have seen I would allow marking every time. I have played with it, and in generally think it is a negative, but I could be swayed if players wanted it, but when I have used it, it has been an overall negative. Flanking has been a huge negative when we have used it.
In games I play as a player it varies, but in general most DMs allow most if not all character build options.
This is something I find irritating on this board. I know it is me and I shouldn't, but rolling is the default method and a lot of players do it. I would venture more roll abilities than any other single method.
It does generate wide variation in balance, yet it has been the primary way to generate abilities for most of the nearly 50 years of D&D and the game did well with it. This is part of why I don't think balance is the holy grail many do.