pretty sure that
Untrusted system file
Files: steamclient64.dll
and "optional parameters" (-steam -eac_executablename "R5Apex.exe")
Are actually two different issues.
1. new steam client isn't in the anti-cheat white list yet
2. new steam client has a bug that is registering the default params as extra optional params.
then when you click "ok" it accepts them and the re-wraps with the default params again so you get
'-steam <approved optional params> -eac_executablename "R5Apex.exe"'
aka '-steam -steam -eac_executablename "R5Apex.exe -eac_executablename "R5Apex.exe'
Edit:
My bro ran into this first, I was able to play apex through steam fine but downloaded the new steam client to confirm that that would give me the same issue. it did.
you can appease the anti-cheat by deleting the specified file (steam will reinstall it on restart) but I don't have a work around for the issue with approving the "optional parameters".
Edit 2: Bro can launch via origin without issue, just not via steam.