[sarabande]

[This is not legal advice]

Not a good sign. Most DE-based eng contracts max at 3 months, or the default per § 622 BGB.

6 months likely means the company fails to retain good staff. Negotiate for asymmetric § 622 BGB notice for yourself, and 3 months notice from the company.

[spi]

I've worked in two companies in Germany and, unless I'm wrong, both of them had 6 months periods for Senior position. At least the second one has no problem retaining good staff.

As several others mention here, this is mostly on paper only. In both companies I've seen people leaving with any amount of notice, between 1 and 6 months. You just asked your manager and, if you have decent relationship, you'd get an okay. If you didn't, there's really no reason for them to keep you doing nothing (which, at that point, is almost a given). The only real risk is if you have a really bad manager who wants to "punish" you for leaving, even if this is at detriment of everybody involved.

At least if the company from the OP is a very large one, I wouldn't assume it to be a red flag, but rather bet on the company having an overdeveloped bureaucracy department, who created central contracts who-knows-why and won't change them even though the practice is always different.