Feel free to adopt this technique for your proposed website slot, so it does not cause compatibility problems. Please do not remove other techniques.
There is no compatibility problem whatsoever with adding optional slots.
I just tried, and got this:
Error while trying to load definition for system wwwoops from pathname /home/xach/src/lisp/wwwoops/wwwoops.asd: Invalid initialization argument: :WEBSITE in call for class #<STANDARD-CLASS ASDF:SYSTEM>. [Condition of type ASDF:LOAD-SYSTEM-DEFINITION-ERROR]There *is* a compatibility problem.
I haven't added the slots yet, so of course it won't work. As for disabling properties on old versions of ASDF that don't actually support them, that's what #+asdf3 is for, just like #+asdf2 before it. Hopefully, two years from now we can assume everyone has moved from ASDF2 to ASDF3, just like today we can safely assume no one uses ASDF1 anymore.
—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org Oignez villain, il vous poindra ; poignez villain, il vous oindra. — Rabelais