In SY-SAPRL 6.20, it's mentioned in the documentation (press F1 on SYNTAX-CHECK).
It's not documented in release 4.x .
In 4.6B I've encountered several bugs when searching for all errors (instead of just the first error).
I didn't check whether all the bugs I discovered in 4.6 have been fixed in 6.20.
this is for an upgrade tool that we are developing and what we need is a rough idea to make the assessment of the system
Are you planning an update?
From/to which SY-SAPRL?
Or do you want to sell a general purpose update tool?
Wich releases do you want to support?
A SYNTAX-CHECK which displays all errors is not at all useable for your purpose, because it's poorly implemented.
(Much worse than what you might expect if you compare the results with the diagnostics of a C compiler.)
There are better options than a syntax check.