GPL effects "derived works". So if your code is derived from proprietary code, you can not use GPL, as you would need to re-license the proprietary code and you can't do that (assuming you do not hold the copyright for the proprietary code). LGPL and permissive licenses are probably fine though.
Now what exactly is a "derived work"? That is unfortunate up to interpretation and different organizations draw the line in slightly different places. We'd need people to go to court to get that line nailed down more firmly.
Governments triggered this entire discussion with their papers and plans to strengthen cyber defenses. The article states that some experts ask for our industry to be more regulated in this regard.
I am surprised that possible regulations are not even listed as a factor that in the decission to stay with C++ or move to something else.
Sure, COBOL is still around after decades, but nobody ever tried to pressure banks into replaceing that technology AFAICT.