So.
Which doesn't in the slightest address that it's the DM's responsibility to write good adventures and give players spotlight, and highlighting features they haven't used is important. Please, actually address what is written.
Really? Do you give the caster a chance to cast? Then yes, you are aware of their ability and have given it spotlight, exactly as I mentioned. Again, I don't think you are actually addressing my point, just making up a point in your head that you are trying to respond to.
Creating adventures tailored to the party isn't something you do at an open table. So this is irrelevant to what I put forth.
Honestly, all of this comes across as... not responding to what I wrote, because you're... assuming I didn't respond to what you wrote in response to this particular nugget of advice:
The DM must know all of the abilities of all of the PCs in the party.
To which you responded, and I quote...
This is actually great advice. If your players pick abilities to focus on things, like trap disarming or what-have-you, and your don't know about it so they never get a chance to shine, that's a missed DM opportunity and makes the player regret picking it.
Now, you think it's bad advice - why do you feel that way?
You
did not include qualifiers such as "but only when you're creating adventures tailored to the party", or "but only when you're not playing a published module out of the box". You included no qualifiers at all. You just straight-up agreed with "The DM must know all of the abilities of all of the PCs in the party." If you want to amend your position to include such qualifiers, well and good, but it's no use trying to rebut my remarks as if you had included them all along. The actual words you responded to and the actual words you wrote are
right there.
The simple fact is that your initial statement was one of unreserved and unqualified agreement with spectacularly bad DMing advice. If you didn't intend to express that degree of agreement, well and good, but it's bad joss to go around pretending that's not what you did.
As for specifics, frankly, I still disagree even with qualifiers such as those added in, so strongly that I'm prepared to say you're just plain wrong.
To my first point, I stand by my statement without modification.
Players are responsible for their characters. DMs have enough to be getting on with. If a DM
wants to generate content specifically catering to their players, well and good. And if, in order to do so, a DM
wants to have a strong handle on what the PCs can do, also well and good. But expecting DMs to "know all of the abilities of all of the PCs in the party" is bad advice, plain and simple.
I want my players to be advocates for themselves (unless they're both younger and newer players). If you pick an ability, and you feel that it's being underserved is causing spotlight issues, bring it up with the DM. Let's also keep in mind that managing who gets the spotlight, which is part of what a DM does, is not exclusively a province of "know[ing] all of the abilities of all of the PCs in the party" - or indeed any of them at all, necessarily. Allowing PCs to leverage relationships with NPCs, leverage social or celebrity status, or achieve particular goals or objectives, or presenting adventure hooks tailored to appeal to certain PCs' goals or personal desires are all ways of managing spotlight that don't require knowing their abilities.
To my second point.
Even 5e, which is lightweight compared to 3.5 or 4e when it comes to player-facing content, has
hundreds of pages of player-facing content, comprising dozens of race, class, background and subclass options, hundreds of features among those options, and several hundreds of spells. By way of comparison, the first-printing PHB is just over 300 pages, or, say, just a few pages short of being twice as thick as the entire ruleset (including optional rules and setup charts) of the most recent edition of
World in Flames, a meaty and complicated wargame where I would say it would be
entirely unreasonable to expect a player to know the entire ruleset. And that's without taking into account monster statblocks, DMG content, supplements such as Xanathar's or Tasha's, and rules and content added in adventures and supplements such as Piety in the Theros book, all the waterborne adventure stuff in Saltmarsh, etc. etc.
Not to mention your rebuttal of my second point is nonsensical. What does, "Do you give the caster a chance to cast?" even mean? Unless I'm going out of my way to shut down spellcasting at every opportunity,
there's nothing I can do to not "give the caster a chance to cast" - they just announce they're doing it and then do it. What is more, knowing that a PC can cast spells is not at all the same thing as knowing
every spell they can cast, and frankly, it's sketchy that you're coming across as conflating the two things.
To my third point.
Since you did not exclude open table games from your initial statements
as they are written, it frankly strikes me as illegitimately moving the goalposts to turn around after the fact and claim that you all along never meant to include them. That consideration aside, given points (1) and (2), suffice to say I stand by this point as well.