I agree with Luke Pinner -- definitely identify arcpy with its 'host' software and version, that is arcpy (ArcMap 10.5.1) or arcpy (ArcMap 10.5.1 x64) or arcpy (Pro 2.3). The ArcMap Python is always 2.x and and the Pro Python is always 3.x.
There are a limited group of tools that are not available in ArcGIS Pro. I don't know of a global list. (Update: There is one in the help!) Most tools are still there, the ones that are missing are related to tools that are simply not there anymore. Some tools have options that have been removed (Calculate Field only supports VBScript in x32 versions of arcpy.)
These missing tools include the ArcMap Coverage tools (since coverages are no longer supported in Pro), the metadata tools (from the ArcMap Conversion toolbox), some other random tools. There may be some geometric network tools that missing because the GM is replaced by the new Utility Network in Pro. Some other conversion tools (Create Personal Geodatabase, Import from Interchange file) are gone because the datatypes are not supported by Pro.
The Metadata GP tools from the Conversion toolbox are not supported in Pro because the XML framework they use is 32-bit only and ArcGIS Pro's Python is 64 bit. (They aren't available in ArcMap x64 bit background processing, either.) There are no plans as far as I know to re-implement these tools in Pro. Last I heard they plan to satisfy the use cases for custom metadata manipulation using the SDK, but I have read elsewhere that they are in the "roadmap". It could be a while so I am glad ArcMap is here until at least 2025. I suggest leaving your metadata update script as-is in ArcMap's arcpy.
Unless you need some really new fancy Python 3 functionality, I recommend coding Python scripts so they will run in ArcMap (py 2.7) and Pro (py 3.5.x) for maximum code re-use. This is possible more often than you think!
... Just like the question title.... I thought I missed a whole generation of dot versions and a major change to boot
