I would like to know how long a route would maintain its community tag when it traverses ASs?
Basically, a route is tagged 100:1000 when exiting AS100, and then accepted into AS200. AS200 will not modify the tag. Would AS200 export the route to AS300 with the same 100:1000 tag?
Yes, the router just passes the community attribute without modifying it from one router to another.
Here is doc with examples:
the BGP community attribute can be propagated without any change or additional BGP communities can be added to the route.
Multiple BGP communities are supported and usually new BGP communities are added.
However, it is technically possible to remove a BGP community when desired, but this case is less common.
You can check if the route still has the desired BGP community using a looking glass.
for a list of looking glasses
In this way you can understand what changes, if any, have been performed by your upstream provider
Hope to help