that not how packaging works, its a build time dependency and we package it, optional is the wrong tool here.
If we want something like that, then runtime optional or split package would make sense.

What is the concern you want to address?

I don't consider Sdl2, vulkan and libplacebo as build time dependencies for dav1d, as it will build just fine without them (meson will build dav1d and will not build dav1dplay). dav1dplay is a sample app. You could consider this an "extra", and is not what 90% of users will want.
dav1dplay is a sample to guive an example to developpers who want to build a player that uses dav1d (it is not a production ready player like vlc or mpv).

My concern is that dav1dplay introduces many dependencies that might not be desirable to some users (embedded system or headless transcoders). I was wondering if it was possible to make these dependencies optional or have some sort of extra package that would include dav1d play.

Just get rid of dav1dplay and those deps. Nobody needs it.

I just made dav1dplay an optional example that doesn't get compiled by default.

Feel free to cherry-pick commit 3a77c57b0ca06c613fea96afe63d2870f513e97f (until a new release is made) so you can get rid of all the unnecessary dependencies.