I don't really know, but my personal opinion is, that most of the I1 came with Germanics, however, like some E-V13 clades, some might have been brought by Slavs coming down from Slovakia-Ukraine-Romania-Hungary and not been there before. But this needs to be tested, especially in Slovaks its hard to distinguish in every single case whether its more ancient Germanic tribal or more recent German Eastern colonisation.
A Romanian subclade like this one could be both:
https://www.yfull.com/tree/I-Y21391/
Or here:
https://www.yfull.com/tree/I-S19185/
Like with all the other haplogroups, we need more samples. I didn't check them on FTDNA, probably they already have closer matches in England or Germany, which would prove the point. Like if having a TMRCA with Germans of around 1.000 BP, you know its recent German. If its older than 1.800 BP, you never know.
This looks like a very old Germanic which might have been spread by Slavs:
https://www.yfull.com/tree/I-Y3568/
But even in this case, you clearly see on FTDNA that the I-S19185 context is Germanic, with only some subclades, like in I-FT244582 being spread in Slavic speakers. This subclade is however fairly old, so could have entered Slavic groups pretty early.