ASDF 2.26.21 removed the horribly clunky :if-component-dep-fails. ASDF 2.26.22 added a much more meaningful :if-feature.
Any component can use the :if-feature option, which can be an arbitrary keyword expression, just like #+. The difference is that unlike #+ it doesn't magically change the *package* to keyword, so you have to include the colons, as in :if-feature (:and :sbcl :x86-64)
The upside is that its semantics is simple, doesn't break the object model, and allows for arbitrary expressions. The downside is that it is not backwards compatible. It will be released with 2.27 only.
(If you have a better name than :if-feature, now is the time to suggest it.)
Hopefully, I will remember to include a #+asdf2.27 before I release.
Please test asdf 2.27 and give me feedback.
—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org The worst thing that can happen to a good cause is, not to be skillfully attacked, but to be ineptly defended. — F. Bastiat