That was the old Microsoft, for all its faults. The new Microsoft is much more Google-like: products are chucked out and then killed shortly after if they get no traction. They deprecate and remove services with ruthless regularity even when they are considered first-rate - like Lync/SkypeForBusiness.
There is a second choice that works more like the Google sheets API...the Microsoft Graph API: https://docs.microsoft.com/en-us/graph/api/resources/excel
When I last tried it, it was somewhat limited in functionality.
Google's spreadsheet API is very powerful, and contrary to the spirit of the OP, I think the update largely preserves existing functionality (the developer may need to adjust their thinking and be aware of other aspects/patterns of Google APIs). I'm actually very glad they came out with v4, as during the lifetime of v3 it was not entirely clear whether they would be updating it further.
Offline spreadsheets are completely unusable for most of the use cases that these APIs enable. The biggest value add of the online spreadsheet APIs is to be able to integrate with relevant online data sources to dynamically update the spreadsheet contents, provide typeahead suggestions/autocompletions, or sync data with another system. This can be very powerful because it immediately brings the full power of spreadsheets to the user interface of your data system, without having to build something equivalent to a spreadsheet interface yourself (which, contrary to some loud opinions out there, can be a huge win in business applications by enabling non-technical or partially technical users to be more flexible, more productive, and less dependent on other members of the organization to do their work).
Until the graph API, the closest thing to a spreadsheet API provided by a Google competitor was... VBA. Visual Basic for Applications. That is still the main API for desktop Office applications. Good luck using that. I just about gouged my eyes out the last time I tried to use it.
I had been looking at using APIs and automation with G-Suite but was always a little nervous to tie my apps to the whims of Google. This is another example of why I shouldn't entertain that thought and use "old, boring technology" like offline spreadsheets (like Libre Office docs) or Office 365 instead.