Jeans in Religious Attractions OK?

Yes, you will see jeans everywhere in Europe. Depending on your age you probably do want to stay away from torn jeans with lots of holes in them.

As long as pants/skirt cover the knees, anything is fine.

It is the first time I have heard of churches called "Religious Attractions".

BlockquoteIt is the first time I have heard of churches called "Religious Attractions".
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As most big churches were built not only to attract visitors, but to out do neighboring cities, and impress foreigners, attraction seems apt.

Yes.

It’s all about modesty, not style/formality. As far as the church cares you can wear sweatpants or a paper bag as long as it covers your knees and shoulders.
Jeans are a better choice - you won’t look out of place at all.

Tidy jeans are our standard attire in Europe (we don't dine at restaurants requiring dress clothes) and we've never had an issue with them. You'll see them EVERYWHERE, just as you do here!

Almost anything is ok in religious attractions as long as your skin is covered above your knees.

The most strict locations are major religious sites, like the Vatican.

Churches are included this list of "10 Top Religious Attractions in the U.S.". This list doesn't even include the "Ark Encounter" in Kentucky: Religious Themed Attractions

Well, pilgrimage was one of the first forms of site-seeing in many cultures. Many large churches weren’t built to hold their local congregation, but the pilgrims coming or passing through. I think church-as-religious-attraction is historically accurate.

They don't mind even well worn jeans as long as they are holey but not torn.

Seriously, we take pants/slacks instead of jeans because jeans usually weigh more.

I agree with HK. Churches have been used in many ways throughout history. Some of the wealthy used them for status. For example, Donated stained glass window with family name on plaque next to it. ( I think the current buzz-word for it is "virtue signaling.")
Some were used as a market place in the middle of the week. Some used as concert halls. Most have been tied to politics in one way or another. Medieval Bishops acted as defacto governors and judges in their regions.
Some were pagan sites taken over by Christians in power politics. (Co-opting.)
Pilgrimages as mentioned above. Along with that, there have always been vendors and businesses that catered to religious travelers. So, as you approach the Vatican and think it is "unholy/annoying/exploitive" to be solicited by tour group operators, t-shirt and souvenir knick-knack sellers, etc. - just remember that these business practices have been going on for centuries and you are having an authentic pilgrimage experience.