This is not a "standard." From their documentation[0]:

  Browsers already know how to interpret certain rel
  values because of web standards.

  ...

  The monetization link type, however, isn’t a standard yet.

And nothing about the "Web Monetization" primary intent[1] is appealing as a standard worthy of being adopted:

  Web Monetization gives publishers more revenue options ...

0 - https://webmonetization.org/developers/link-element-webpage/...

1 - https://webmonetization.org/