Either I'm blind or there seems to be a severe lack of request join button for guilds.

In what context, Exo? Can't you request to join a guild while in another guild?

Or have you got an invite but can't click join?

I believe I left my previous guild and then tried to search for it on guild's page, but didn't find any, had Symphony send me invite and joined through there.

Hmm, I'll take a look.

Edit: Yep, confirmed. I can only request to join a guild if I'm in one already. How on earth did that regress. I'll fix today.

Edit 2: Fixed