I don't feel I should have to tell people to stay behind, but that is how BG1 & BG2 handled it.
Techincally not quite - in BG1&2 you had a party and that's it. No camp for your spare companions hanging out, waiting for you to progress your storylines like lost puppies. This is probably my favourite way of doing it - I don't need to experience every companion in one playthrough. RPGs are about choices and building party should be one of them.
On the other hand it is understandable that as more resources were sunk into companions, the less comfortable devs were on player missing out on majority of them. I think it is case by case basis - if there is a narrative reason to "Gotta-catch-them-all" companions, and if swapping them around is naturally, it can work really well (usually it's some sort of ship to which we return, though BG3's camp theoretically works, if we ignore the weird nature of the pocket dimention camp itself).
Specifically to your point though - yeah, I find it weird. More in terms of how unnecessary clunky it is (especially with an inability to access inventory of companions not currently in your party), then making me feeling bad, but I never liked it when NPCs bring attention to artificial constrains of the game. Like "Oh, your party is full, I can't join you". Like really? There are four of us. Why is 5 too many? Either you can integrate mechanics with narrative or you can't. Don't pretend that you do.
BG3 would simply be better with some kind of party UI, but I suspect they just aren't willing to invest resource into creating a completely new UI, and opted to do it through existing system - dialogue. Though if spare companions are too be removed after act1 that would even make some amount of sense.
