There's nothing like official TFS 1.5 for 8.6.
1.5 is simply
master
branch of official TFS (under development) that's currently for version
12.80 - 12.88.
There are some downgrades of TFS to older protocols, just look for some downgrade that's actively developed or make a fork of some and mantain it on your own by aplying some fixes from official TFS and ofc. your changes.
Here's one that looks promising as it's still actively maintained (but still not a complete solution to download & host):
TFS Downgrade 1.5+ is an engine based on nekiro downgrade but highly updated with current tfs code, it also has the lua modules divided and uses lua5.4 - MillhioreBT/forgottenserver-downgrade
github.com
In the era of OTClient I kind of don't get why do people want to downgrade to older protocols rather than just use latest one and disable some features that they're not interested in. Only reason that I see is to let players use official old client and bots.