In 2020, Usernames are now hidden as long as you have a display name, your username will still be shown if you don't have a display name, however, this feature was removed sometime in October 2020 then re added back sometime in November 2020. However, during an unknown date, this feature was somehow available for PC users as well. This feature was added between early June or July 2019. In 2019, a new mobile feature made it so that when you make an account on mobile, you can have the username "Robloxian" consisting with 9 random numbers right after it.
Currently, Roblox usernames can consist of 3-20 characters using letters, numbers and a single underscore (_) which must not be the first or last character of the username. This was to prevent users from creating usernames of phone numbers and because Roblox was adding a login by phone number feature. In 2018, users were prevented from having usernames made up of only numbers. On February 24, 2016, underscores were added back into the allowed list of characters in usernames (the first time since they were initially removed back in December 2006). This feature was removed sometime in 2013 or 2014.
In the early 2010s, if a username you wanted was taken, Roblox would add a randomly generated word to the first half of it (as a suggestion for the user) to make it untaken on the sign-up page. During this time a few administrator accounts had periods in the username, this feature was likely only included during Roblox's beta period between 20.
#OG GAMERTAGS NOT TAKEN 2018 FULL#
During the early stages of Roblox, usernames could have spaces in them, such as John Doe and Jane Doe, but this feature was later removed on December 21, 2006, due to users impersonating other users by using spaces, and using their full names to create their username.