[dspillett]

There is an open source engine offering similar features that might be a better candidate for other products to integrate with - https://www.languagetool.org/

I've been meaning to have a play with it as a potential replacement for Grammarly before the special offer subscription of that is due to renew in a couple of months (though if I keep the special offer rate it is probably just as well for me to stick with Grammarly).

It is Java based which may put off a lot of projects from including it directly in their desktop or mobile products, but they could host their own instance and have their applications submit data to that. Assuming the extra admin (keeping the service running & up-to-date, and monitoring/enforcing API-key use if they don't want the service to become de-facto public once other developers notice it exists) and hosting costs would not be too high, of course.