A recent update to DOTA 2's text files has been found to include a file labelled hl3.txt and within it mention of ziplines, squads, citizens, NPCs and VR.

Naturally, this has led certain portions of the internet to completely lose their shit that Half-Life 3 is definitely happening. Believe, etc.

The files in question are here and as noted by SteamDB include:

"Element.prop_zipline"

"npc.quest/citizen" "If you want your citizens to squad with the player but not obey commands the player gives to move the squad, put a concept here and this citizen will speak the concept instead of obeying the order."

"vrchaperone" and "vrmovement"

"npc_turret_ceiling_pulse"

Whether these files are something genuinely Half-Life 3 related, we can only speculate. It could be a hoax, an accident, old info, a new leak or one of many other possibilities.