The availability of IRS transcripts, depends on the type of transcripts requested.
Wage & Income transcripts, which are the items reported by 3rd parties, W-2's, 1099's, etc, are available for 10 years. (There is no state info on these, such as state withholding tax)
Account transcripts, which report the the balance due, penalties, payments, interest are available for at least ten years, and indefinnitely if there is any balance due.
Return transcripts are only available for the last 3 years. This would include late filed returns that were filed in the last 3 years.
Record of Account - combines the Account Transcript with the Return Transcript, but, again, is only available for the last 3 years.
The IRS has a lookback period of 6 years, with a cutoff at the end of May each year. So, the returns that they would require now are 2013-2018. Next May 2014 will fall off. I have had clients that wanted to file returns older than 6 years, but the IRS will not accept them and will mail them back.