Saying that It only works on 13.4.1 seems like a little more than a stretch... of the reality. Cant speak too too much about any of the IOS 12 versions but
Apple has had for IOS 12 - 12.0.1, 12.1, 12.1.1, 12.1.2, 12.1.3, 12.1.4, 12.2, 12.3 12.3.1,12.3.2, 12.4, 12.4.1, 12.4.2, 12.4.3, 12.4.4, 12.4.5, and 12.4.6 as far as their versions go..(not mentioning whatever current versions is out for IOS 13 and.. everything in between prior to IOS 12... Now, Figuring out if someone has a specific IOS version is a little more than tough, but if you have an IOS version where this doesn't seem to work or it seems that YOU know the target has an Idevice with a hunch that he may have a version in which it doesn't seem affected(meaning anything below IOS 13.4.1..., either that or they know how to fix it lol and don't bother complaining(on the first encounter with the text bomb unlikely).. but
If they are on previous versions of IOS like say 7-8... and through 11 perhaps, Youtube is filled with Links that crash iphones.. Remember Effective Power? god those were good times.. anyway I was doing some research and IOS 12... as far as exploits go.. like it was a moment of dead silence for this kind of thing, even the Black hole text bug got patched in IOS 12. The funny part is, the more text bug/bombs they find the easier it will be for older versions to be used/rekt for this kind of thing. I forgot to ask in the beginning of this Ramble... to tell me.. What is the SPECIFIC IOS version You are on, when you used this?(Go to your settings in find out cause "just IOS 12" is a bit vague, thanks). So that way I know what i'm looking for. That way, perhaps You can send 1 text with like 5 different text bombs/bugs crashing links from now and the past Just in case for those Pesky people that just don't go down on Idevices.