Similarity checker is currently available in Office Preview builds in Microsoft Word for Microsoft 365 EDU A3 and A5 customers. Microsoft plans to make it generally available to all the Office 365 users in July with the exception of Consumer and Enterprise users who will get the feature later this year.
Got an update to version 1.1.5 today with one massive improvement:
With the previous version you must manually switch languages (English and Dutch for me), but with this version you can select up to three proofing languages.
So, no need to switch manually anymore