A little late to the party here but might be an interesting data point: Flying ORD-FRA on UA (connecting to VCE on LH) in March, the purser made a very dire sounding PA about passports must be signed, and if not signed yet the time you get to immigration you'd face a 1000 euro fine and be refused entry.

My passport has a variation of my usual scribble (you can make out the first few letters of my first name but it includes only half of my hyphenated last name because...well it's a long story. Of that maybe 1 character is recognizable... yet Germany didn't bat an eye. I'm guessing their acceptance criteria ls along the lines of the average POS signature pad (incidentally, I usually just make a horizontal-ish line, I don't even bother scribbling anymore and have a never had an issue)
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