A fairly niche Office 365 plan, “Skype for Business Online Plan 1” included group IM, ability to transfer files in IM, audio and HD video calling to Skype for Business users but no Office 365 application licences.
It will be retired, effective August 1 2018. After that date customers can no longer purchase new Skype for Business Plan 1 subscriptions, and existing subscriptions will not renew automatically when they expire and will not provide a renewal option.
Both Skype for Business Online Plan 1 commercial and government SKUs are retired.
Microsoft’s recommendation is to move to F1 plans or other Office 365 plans
Not the best choice of URLs all things considered, IMHO