I lived through many transitions in dress of the Pentecostals. And dress particulars were primarily for women although short pants was frowned upon for men. My mom became a Christian back in the days where Pentecostal women did not wear makeup or jewelry and wore hats to church. I recall the first time women started to wear a broach on their dress. How worldly ! As for style of dress for women, dresses had to go below the knees in length and have sleeves and no plunging necklines. There were no written particulars but just what was acceptable was shared and sometimes preached. Women were not to cut their hair higher than shoulder length. Life style had various norms to keep away from as they were considered worldly for various reasons. We did not play sports on Sunday; go to county fairs; no dancing'; suits and ties for men when going to church and various other acceptable norms were followed without any of this written somewhere. it was more of what we thought was living 'sanctified'. I never heard the word 'plain' used but rather either living 'sanctified' or living 'holy' were the words used.
Today very few of these practises are followed in most Pentecostal churches, that I know of. My guess is that 70 years ago the Pentecostal lifestyle was quite close to the Mennonite Brethren lifestyle with a few exceptions. One exception back then was women did not sit separate from men in Pentecostalism and they were allowed to preach, teach and be pastors. But today the MBs and Pentecostals would not have any noticeable differences. As far as dress for church, one could wear most anything other than a bathing suit which likely would be challenged. For headcoverings, nothing is required but some young men even wear ball caps with various logos and/or writings. Headcoverings are considered a personal conviction item and if one choses to wear one they are not treated differently.
When I think of dressing modestly, if we really want to be more scriptural, I would suggest Christians should all dress in robes as they did in Jesus time. That is how the early church dressed and how scriptures speak of the dress type when we get to heaven, that is if one takes scripture literally. If we all just went back to robe wearing and kept a white robe for church gatherings, that, imo, would solve considerable debates regarding dress and would fit with my view of being 'plain' where we don't stand out from one another. As for make up and other adornments (jewelry of sorts) that would be unacceptable. Men wearing robes would solve most issues with what men wear. Women could have robes with hoods for church for those who believe in covering their heads and there would be no debate on what is an acceptable headcovering. Personally I would be attracted to such a church. What they wore outside of church would be a personal conviction area but I would love to go to a church where the distractions of adornments was basically removed. Now that would be radical Christianity, yes/no ?