An useful feature would be setting permissions/groups/prefixes/suffixes until a player leaves the server

I WOULD say that this is already just default functionality because if they are not on the server, they do not have permissions. BUT, what I think you mean is that you have permissions based on your current session. When you disconnect, you lose whatever session perms you were given. Interesting idea.

@lucko I was thinking, if someone wants to give players the possibility to join with a custom client or something like that, you could identify the players who use it

This can already be achieved via the Bukkit API :)

https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/permissions/PermissionAttachment.html

What are the possible usecases?

Yes, but this would be an useful feature

Yes, but I don't see when you'd ever need to assign "transient" permissions via a command call - and therefore don't think it makes sense for a whole new set of commands in LP for it.

Such assignments can be made programatically through the server API, if you want to assign perks for custom clients or that sort of thing.

it may be useful to integrate this feature with plugins that do not directly support this feature but can execute commands

I'm not so sure, I think it would make more sense for it to be written into a separate plugin.

Thanks for the suggestion, but I don't think it's appropriate for LP