I don't know if any of you have encountered this, but I'm unsure if this is a bug or not. Recently, I tried to change my Summoner Name to another that I desired, which turned out to be "Unavailable." According to Riot's Summoner Name FAQ, "Each Summoner name will have 6 months of inactivity protection upon creation.

• Each Summoner name will earn an additional month of inactivity protection for each Summoner level above 6, up to 30 months.
• Inactivity is defined to be a period in which no games of any kind have been played on the account.
• When inactivity protection expires, your name will be available to be claimed through the Summoner name Change service in the store."

From this, the name that I wish to change to should be available. I asked some friends about this and they came back to me with the same result. Furthermore, one even commented that they actually do not mark some names for inactivity, which I do not know to be true or not. I will be contacting support after this post.
I also used this website to check the availability of the name. It's not 100% accurate as it's not affiliated with Riot and a third party, but usually it's correct. I claimed another summoner name using this same website.
Another thing is, the mastery system was implemented back in 2015, I believe. The account currently holding the name I want, does not have any mastery registered. I also don't know if this a bug or whatnot, but I'm only pointing out what I can see. Please let me know what you guys think about this, and / or have experienced this. I would like to see to it that I could get my desired name.