It is often useful to check and see how close the "next" short domain type is to being bought out, and how the domain type is trending.
4 character (4C or CCCC.com's) are hot right now, and are being registered at a steady pace. There are approximately 1.2 million 4 character domains in total. They are still far from being all bought out (another 5-7 years perhaps), but here are some recent data points to give an idea of how popular they are:
April 16, 2018: 614,877 registered
August 19, 2019: 664,451 registered
6 number (6N or NNNNNN.com domains) are also being registered at a strong pace. There are 1 million total, and these are the closest to being bought out (again), perhaps only a couple years until the 6N buyout:
April 16, 2018: 771,128 registered
August 19, 2019: 796,854 registered
As for five letter domains (5L or LLLLL.com's), these are far from being bought out. There are 11,881,376 total of these domains, and we only have a couple data points here:
April 16, 2019: 3,257,718 registered
August 19, 2019: 3,156,254 registered
Registration's for 5L's actually declined in the last few months which was surprising. We will keep a closer eye on these domains. There is not much chance there will be a buyout of 5L's for decades (if ever), but it is still interesting to see the numbers as a bell-weather for short domain, and letter registrations, in general.