It is not illegal, in any situation, to _apply_ for asylum. It is up to the government to evaluate. If granted, it is inherently legal. If not, well, still not illegal to apply for asylum.

> Something like 90+% of claimants disappear after their initial catch-and-release

Citation needed. The claimants who are eligible for release are ones that _CBP_ deemed low risk, and are fitted with ankle monitors. They're also only released in the first place because we make very little (to now nearly zero) effort to process them in a timely manner.