Was an uppercase j used to replace an uppercase i letter on German monument inscriptions?

The convention of replacing uppercase I with J seems to be used in other languages, too. I recently visited a WW1 war grave with a monument commemorating Italian POWs:

Agli Eroi d‘Jtalia

1914-1918

Italian does not use the letter J in native words, so replacing I with J shouldn't be problematic regarding pronunciation.

Actually it is much simpler: the name Ida is sometimes written Jda; the inscription says Jda because the woman wrote her name that way.

This does not mean that in general one can substitute the letter J for the letter I in German. It is a peculiarity of the name Ida.

• A semi-well-known example is the Prospect vom Berge Jda (A view from mount Ida), engraved by Albert Henry Payne after a painting by William Henry Bartlett, c. 1850). (The Mount Ida in question is neither of the two classical Mount Idas, but rather some hill in New York State, U.S.A.)

• Here is an article from the newspaper Entlebucher Anzeiger, dated 20 June 2014: "Ski-Weltmeisterin Jda Schöpfer verstorben", ski woman-world-champion Ida Schoepfer is dead. (Note that the article has both Jda and Ida. Newspapers these days.)

German Wikipedia has this to say:

Bis heute gibt es noch ältere Schreiber, die anstelle des Großbuchstabens I ein J verwenden (z. B. Jda, Jtalien). Auch bei serifenlosen Schriften wird manchmal ein großes J anstelle eines großen I gesetzt. Ein Grund dafür ist, dass bei derartigen Schriften das große I und das kleine L oft schwer oder gar nicht unterscheidbar sind, vor allem wenn beide Buchstaben nebeneinanderstehen (etwa in Jll, Jller, Jlmenau, Jllustrierte im Unterschied zu Ill, Iller, Ilmenau, Illustrierte).

Bei einigen Fremdwörtern existiert sowohl eine nach der neuen deutschen Rechtschreibung gültige eingedeutschte Schreibweise mit J, als auch eine fachsprachliche mit I (z. B. Iod, neben Jod). In der Chemie wird die Schreibweise mit I sogar bevorzugt.

Translation:

To this day there are older writers who use a J instead of the capital I (e.g. Jda, Jtalien). A capital J is sometimes used instead of a capital I in sans serif fonts. One reason for this is that in such typefaces, the uppercase I and the lowercase L are often difficult or impossible to distinguish, especially when both letters are next to each other (e.g. in Jll, Jller, Jlmenau, Illustrierte as opposed to Ill, Iller, Ilmenau, Illustrierte).

For some foreign words, there is both a German spelling with J, which is valid according to the new German orthography, and a technical spelling with I (e.g. Iod, next to Jod). In chemistry, the spelling with I is even preferred.

Can uppercase j be used to replace an uppercase i letter on a German monument inscription?

Before 1945, it was common place to replace uppercase 'i' with an uppercase 'j' letter.

The main reason was that in the past a uppercase 'I' was written (in the Antiqua type set) as a vertical line (without the now common horizontal bars on the top and bottom) and thus looked the same as a lowercase 'l' that was also written as a vertical line.

Uppercase 'J' was therefore used to make the distinction clear.

J - Verwendung in der deutschen Sprache – Wikipedia:

• Jll, Jller, Jlmenau, Jllustrierte
• Ill, Iller, Ilmenau, Illustrierte

If you look at the bottom of the sample image, you will see 'Dipl. Ing.' (where 'J' is not used and is dated with 1978).

It looks the same as the 'l' of 'Dipl'.

Even when Fraktur was used, the uppercase letter 'J' was often used instead of an uppercase 'I'.

Berliner Illustrierte Zeitung Archiv 1895 - 1945 | Historische-Magazine.de

1895-09-22	1912-04-28

Berliner Illustrirte Zeitung - Wikipedia

Also in 1941, the old-fashioned spelling of its name (sometimes described as a mistake), which had been retained when the masthead was modernised at the turn of the century,
was finally changed to the more modern Illustrierte [from Illustrirte].

• the usage of the uppercase letter 'J' was not changed

Fraktur, a typeface that was common in Germany until well into the 20th century, uses a capital I that is very much alike to a capital J in other fonts. See these samples on Wikipedia; the capital J has an extra dash:

As others pointed out in the comments, this is actually common to other writing scripts, also because a capital I can be very similar to a lower case l.