I see that there is an Open Collective for Castle Game Engine but I don't see it listed on your home page. Is it still an active and valid way to donate ?
Yes, donating through https://opencollective.com/castle-engine is welcome. We have a few ways to donate, listed on https://castle-engine.io/donate_other.php . On the main engine WWW page we focus on https://www.patreon.com/castleengine as this is our preferred method for donation, but all alternative ways listed above are good as well :) Thank you!