Most village signs I’ve seen carry the names of village, tambon, amphoe and changwat making it easy to gather them. The is_in tag was described as depreciated in the wiki when I started to tag villages. The addr:* tag was used for places in other areas already. It’s use for places is controversial because you can argue that you can send a letter to a house but not to a place. I documented the extensions for the Thai addressing scheme on the Thailand project and the Key:addr pages.
I prefer extending the addr scheme with the English terms hamlet, subdistrict, district and province. The terms muban, tambon, amphoe, changwat are Thailand specific and can’t be used somewhere else.
I would never tag a highway with addr. Highways can be dividers in villages with more than one muban.
All this is_in tagging is just a workaround for missing administrative boundaries, right?
If you have the boundary of a tambon, you can easily query everything that’s inside and say this IS IN that tambon.
I also strongly vote for not using Thailand specific keyword. This would break all internationally established software.
according to taginfo the addr: tagging is also used in other places.
For streets this is not needed. Try tagging name, name:en, name:th so multilingual maps can be rendered.