The function CONVERT has a lot of units, which are deprecated in ODF 1.2. LibreOffice should better not allow these units in new documents and at least warn users in the help, that these units are deprecated.
A special warning is needed in case of the deprecated unit "at", because "at" is treated as "atm" although it is a different unit, see bug 72174.
So my suggestion for immediate improvement:
Change the help. For example remove such units from the regular list and add the information, that these units should not be uses although LibreOffice can handle them. Add a dedicated warning for unit "at".
In the long term LibreOffice should get a feature to show warning indicators at cells. But that is a different issue.
Just personal thought: it's a real shame (I'd say insulting for those who work on the concerned domain) that not only Excel but also OASIS was wrong about "at". Oasis should should check units on real sources, not compare with what Excel does.
Now this symbol is lost forever (see https://bugs.documentfoundation.org/show_bug.cgi?id=72174#c26).
Hope there won't be others.