Is there a way to only allow SQL Server 2014 syntax/functions to be used in SQL Server 2019?
I'm wondering if there's a solution to just allow 2014 syntaxes in 2019.


No. If you need your users to steer clear of things that won't work in versions newer than 2014, then they should develop on 2014. 

At the very least, you could let them continue developing on 2019, but automate scripting their objects to a 2014 instance. You can't do this with backup/restore or detach/attach, but there are plenty of ways to transfer objects/data to an older version. You'd also have to run all of the code, because some problems won't crop up until runtime.

Compatibility level is not the answer here, except for very rare exceptions (like STRING_SPLIT). Compatibility level is not used for allowing / disallowing syntax, in modern versions at least. See this and this. In older versions, yes, a lot of syntax was controlled this way, but that’s not how it works anymore.