just based on when it was committed (https://github.com/inaturalist/iNaturalistAPI/commit/181eb83d86f6083e2131497dd8726132677033a4), i think it’s fair to assume they added the parameter specifically for the year in review link, as you described.
there are others – CalFlora, Atlas of Living Australia, etc…
some of these are listed here: https://github.com/inaturalist/inaturalist/blob/main/app/views/shared/_outlink_sites_js.html.haml, but then there are others llike the Maryland Biodiversity Project that aren’t listed there. so i’m not sure if there’s a complete list anywhere in GitHub.
i think a site curator should be able to update that page, but then there’s also https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki-part-1-of-2/63, and really, the place where it really should be documented is https://api.inaturalist.org.
right now, you’d have to dig into GitHub to find it in some version of a parameter list: https://github.com/inaturalist/iNaturalistAPI/blob/main/openapi/schema/request/observations_search.js or https://github.com/inaturalist/iNaturalistAPI/blob/main/lib/models/observation_query_builder.js.