Is this unacceptable

[nickiredcar]

To invite people out for dinner to celebrate a milestone and pay for everyone but to limit it to two courses?

It's fine, my mother regularly does this and is insistent on which two course we may have as she doesn't want people hanging around for a main when two people have ordered a starter.

YABU
If they're paying then they can do what they want.
If you want extra you pay

Sounds pretty generous of them to pay for everyone to be honest.

I think the OP is the one doing the paying, not a guest.

YANBU but I'd probably contact the restaurant in advance and get a set menu printed for the table. Someone I know did his for a fiftieth and it worked really well so he had a menu on the table with options he had picked out for each course and guests could choose from that. It avoided any awkwardness.

(I always want a main and a pudding rather than a starter and a main.)

Sorry OP I misunderstood the gist of what you were trying to say
YNBU at all

It sounds fine to me. Just make it clear in advance.

Yes, OK with me. Just announce in advance what the deal is ... that you're not having starters (or whatever) and going straight into mains and desert ...

It's fine. When we eat out we only ever have two courses, three is too much to eat or pay for.

God no totally acceptable, and lovely of you if that's what you're doing!

The set menu idea from a pp is an excellent idea. Happy birthday!