[banhfun]

It's always been prevented in Dota 2. Buildings have backdoor protection, which makes them regenerate lost HP from recent attacks unless there are creeps nearby.

[ryanlol]

Not really "prevented", backdoor protection only provides 90HP/s.

[Smaug123]

Backdoor protection also gives a substantial damage reduction - 25%, and much more against illusions.

[darrenkopp]

T1 towers don't have backdoor protection though.