I think some optional/variant rules are OK in the
Player's Handbook. In the 2014 PHB we have (just a quick run through -- I may have missed some):
- Customizing Ability Scores (point buy rules)
- Variant Human
- Variant Backgrounds for Criminal, Entertainer, Guild Artisan, Noble, Sailor
- Equipment Sizes
- Skills With Different Abilities
- Variant Encumbrance
- Multiclassing
- Feats (just in a technical sense -- I agree that most games are going to use Feats, so they are optional only "in theory")
If the DM decides these rules are in use in the campaign, the players need to be able to reference them for character creation and ongoing play, so they belong in the PHB, IMO.
D&D is a complex game, for the sake of simplicity, even variants I strongly prefer, I prefer to be in the DMs Guide. It is better for the game for there to be clear mechanical rules, and then when customizing to taste, a clear agreed-on modification of them.
Abilities − use arrays − most experienced players will consult the DMs Guide for either dice rolls or point buy, without privileging either.
Variant Human − there is only one 2024 Human Species with a default or choosable feat − there is no Variant Human.
Backgrounds − the setting is responsible for new backgrounds, including a homebrew setting.
Equipment Sizes − Size is an important mechanic and needs to be well designed and consistent.
Skills with Different Abilities − make the default − Stealth "typically" uses Dex, but example, DM might call for Wis to Perceive onlookers.
Encumbrance − pick the most popular (or in this case the least unpopular) option and put any other options in the DMs Guide.
Multiclass − I dont multiclass, but 2024 looks to be making it default (judging by its sidebar in each class). Fine with me.
Feats − all feats are default, with the option of choosing an other one instead. Ability Score Improvement is the default feat.
It is fine to include every Variant in the DMs Guide.