I've noticed a lot of other communities have speedrunning discords and I haven't been able to find one for this game.
Portal Discord: https://discord.gg/ZkcJJg2 (don't expect anything super active here)
for Source Games in General: https://discord.gg/sourceruns
Yeah your best bet is the sourceruns discord. It's pretty active and people are always willing to help!
Portal-related discord servers: Portal Speedrunning SourceRuns SourceDoneFR Innate's Discord Some other secret discord channels
It was recently discovered that it is possible to set your mouse sensitivity to a value so high that the game considers it infinite, leading to several new glitchy techniques being discovered.
The rules have now been amended to ban these new techniques in both Inbounds No SLA Legacy and Glitchless.
