What computer keyboards are in use in Poland?

On typewirters they were called key-slugs. But in general you are right -- 214 was patterned on the German keyboard. So were many typewriters, however the four German keys Ä, Ö and Ü as well as ß were not sufficient to accommodate all 9 Polish accented letters. £ and Ż were usually separate keys, but that would leave only 2 keys (from the German foursome) for ą, ę, ć, ń, ó, ś and ź. In my opinion the best layouts (Olympia, Adler, Mercedes, Remington, Royal, etc.) had the lower-case accented letters in the top row with the numerals above. Then you could type Polish quickly and efficiently. Upper case were made with a dead key. The PRL-style keyboard was poorly designed and requried shifting for lower-case ę, ś, ń, ć and ź. Instead of smooth typing the typists was constantly shifting. 214 is only a slight improvement as you still have to shift for ć, ę and ź. One of the worst Polish keyboards was on a pre-war Hermes Rocket -- lots of shifting!

Is it true that there existed layouts which were lacking of the uppercase Polish-specific letters - at least of those which theoretically don't exist in uppercase (they don't appear at the beginning of any word - like Ę, Ą) ? I have seen texts where in the parts written in uppercase the Polish signs were lowercase.

The problem is that many typewriter keyboards seemed to have been misdesigned by clueless engineers or maybe elves who had never done any real typing themselves in a given language. A properly designed keyboard should take a given country's typing needs into account.

For instance, all seven upper-case Polish accented letters can be dealt with by a single dead key: '/˛.
I have heard of and seen a picture of only one typewriter from before WW1 that actually had a full (upper and lower-case) Ę and Ą key on the keyboard.

I wouldn't say so

Polish 214 is actually a very terrible implementation of the old Typewriter layout. Its resemblance to the actual typewriter layout is very loose and diacritic letters (ż, ń, etc.) are in a very suboptimal places. Only Ł and ą/ę are in the places, that typewriter layout used. Rest is completely janky. Why doesn't ż have a separate key? Why does ó/ż occupy the least comfortable to press key? Why there's no way, to type in capital letters except the Ł?

TBH, I'd really advise against using it, even the US QWERTY is better for typing in Polish. if you want a good typing experience, either use Macintosh Polish layout or Use some custom community-made layouts like this one: nowapolskaklawiatura.wixsite.com/main/