Are there any (around 14) hidden characters in the path and/or filename in File Explorer?

This is a limitation applied by Explorer itself, not Windows. The same limitation also exists in Windows 10.

Without access to the source code, any attempt to answer this question would be purely speculative.

My hypothesis was that it has to do with Explorer adding “ - Copy” when a copy of a file is created in the same directory as the original. If a copy of a copy is created, the suffix changes to “ - Copy (2),” and so on. I thought that the remaining fourteen characters were reserved for this suffix—you know, some localized variants may be pretty long, and if it supports more than one hundred copies, this means that the digit could take three characters or more.

After some testing, it appears that this is not what happens. I started by creating a directory containing 220 characters (221, counting the trailing backslash)—this way, we can do our tests with relatively short file names. Then, I created a file with the longest name Explorer would allow me to do:

1234567890123456789012.txt—total: 247 characters.

(Same as you, but you got it off by one: the backslash between the directory and the file should count as a character.)

If I do Ctrl+C, Ctrl+V, I get:

12345678901234567 - Copy.txt—total: 249 characters.

By making another copy, I get:

12345678901234567 - Copy (2).txt—total: 253 characters.

Notice how the file name gets cut. Copying the file over and over to reach the counter of one hundred gives, indeed:

12345678901234567 - Copy (100).txt—total: 255 characters.

(and no, I won't continue until I reach a thousand, ten thousands, and one hundred thousand—the life is too short for that).

As “ - Copy” has a different length in languages other than English, let's try with Greek. It appears that the name is cut even more when the suffix is longer:

123456789012 - Αντιγραφή (2).txt—total: 253 characters.

I can go back to a copy and rename it, leading to:

12345678901234567890123 - Copy (2).txt—total: 259 characters.
123456789012345678 - Αντιγραφή (2).txt (same).
1234567890123456789012 - Copy (10).txt (same).

That's very close to the 260 characters limit. I would suppose that developers kept the remaining character for the trailing zero byte. Now, as I increased the directory full path to 240 characters (241, counting the separator character), I could create the following file inside:

12345678901234.txt—total: 259 characters.

And then, I could also copy it, leading to:

12345678901234 - Copy.txt—total: 266 (!) characters.
12345678901234 - Copy (2).txt—total: 270 characters.

There seemed to be no particular issues with this file. I could open it, rename it, and delete it without problems, which might give a hint that the actual limit is mostly useless in modern Windows, as Explorer seems to be using long paths under the hood anyway.

Likely it was implemented a long time ago, as a quick and dirty solution to handle most of the cases with long paths back when those were not supported by Explorer. Not a bad attempt, after all, given that there is no way to handle all the cases anyway. A user can always rename a parent directory to be longer, reaching the 260 characters limit for one of the children.

Now, there is one thing that worries me: I don't speak a word in Greek. How do I get Windows interface back to English?! That would be a good question to post on SuperUser!