We visited Angor Wat and discovered new rules about what you can wear to view the 3rd level. They had an official at the stairs to the 3rd level making sure everyone had their shoulders covered by wearing tshirts/shirt/jumper. If you had a strappy top/bare shoulders you would not be allowed up, even if you covered up with a scarf or a wrap. Our guide found out this rule had started on 1st Nov, and was because some people would cover up with a scarf at the bottom, then when they got to the top, would take it off again.
We saw lots of people being turned away, even when they offered to cover up. The Guides did not seem aware of this new rule. We did not see this rule being enforced at other temples.
My advice would be if you are wearing a sleeveless top, take a tshirt to put on before you get near the 3rd level.
I hope others find this helpful!