Maybe instead of splitting that thread, the portion of posts could be measured just by “the portion of posts”, i.e. the number of posts in threads for LOS compared to the number of posts in other threads.
There are at least three LOS threads (in #software:dev and #software:alternative-oses with really high viewing count and frequent posting:
Porting LineageOS to FP2 193 posts (since April '17); developing.
LineageOS 14.1 with OTA updates for FP2 122 posts (since June '17); developing.
Using LineageOS on the FP2 as mentioned 301 posts (since May '17); alternative OS
That amounts to 616 posts since April 2017; seems quite a lot to me.
Maybe this rough estimation could do it to decide if a LOS-category is needed.
My guess would be, that posters are more likely to open new threads, when there is a special category for LOS.
If it is necessary and worth the effort for the moderators or admins to split up these long threads is beond me. I am here just from a organisational point of view, as I lack all technical understanding for the topics at hand.
And maybe this thread could be counted as well (although it might focus more on cyanogenmod):
Fairphone 2 and CyanogenMod / LineageOS 191 posts (October '15 to April '17); developing.