"Bottleneck" is a term that gets thrown around too often.
A 2600 is a solid pairing with a 1080TI.
Sure, a CPU with a stronger IPC could probably achieve slightly better results in some instances, depending on the resolution, that doesn't make the 2600 'bad'.
Every system has a limiting factor. What it is, will vary depending on specifics (resolution, in-game settings, refresh rate etc) and won't be the same in every game/workload.
IIRC, a 1080TI can push >200FPS at 4K max settings in CS:GO. So for that title, your limiting factor is going to be your monitor before anything else.