It's not exactly a REST API, but for Confluence you can do a POST request to /rpc/json-rpc/confluenceservice-v2/getServerInfo and get the version information.
I am not aware of any API call for Crowd that returns the version number.
You will find Crowd server version in the X-Embedded-Crowd-Version response header that is set by Crowd after any successful REST request. For instance, if you run
curl -u 'test:password' -I 'http://localhost:8095/crowd/rest/usermanagement/1/config/cookie'
You will get something like
HTTP/1.1 200 OK
Date: Sat, 20 Jan 2018 15:20:13 GMT
As far as I know, there is nothing similar for Confluence Server. You might want to vote for this open feature request: https://jira.atlassian.com/browse/CONFSERVER-43007
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