Several simple options:
Eject the drive. If there's nothing to overwrite, the last Time Machine backup will persist there for years.
Copy the entire backup to a different drive that you can put away. You might even be able to burn it to a disc (truly Read Only) or copy to a USB stick. Then, when you want to revive it sometime, copy it back from that source. This option would let you "have your cake and eat it too" in that you would basically keep a copy of this backup as is AND be able to continue making new Time Machine backups going forward.
Turn off Time Machine. If you ask it to no longer do its thing, the last backup persists until you turn Time Machine on again. Of course, this means that anything you create or add to your system won't be backed up at all, so choose wisely here.
Lastly, go buy a smaller drive and then immediately TM backup your Mac. When it is done, eject it and it will be your current copy that will persist for upwards of years on an unplugged drive. This is basically the same as the first suggestion except that you are making a fresh backup to the new drive instead of copying the existing backup to another drive.
If this is about creating a way back ahead of potentially upgrading macOS (and finding something is not working or knowing that some software you need will not work), I suggest a full clone of your drive using Carbon Copy Cloner or Super Duper, making that a fully bootable external drive and then going ahead with whatever you are doing. I have a few external drives to let me boot an old Mac into several versions of OS X and macOS, so that I can go back and use some depreciated software when needed. IMO, this is the BEST way if this is your situation.
If money is tight and another drive is not doable, you could partition the 16TB with enough space to clone your boot drive, use one of those apps to duplicate your boot drive to the new partition and then you will have probably 15TB or so left for normal operations and around 1TB (in the new partition) as a bootable option into the current macOS. The partition could be whatever size your boot drive is now- I'm simply assuming 1TB in this example. Time Machine will basically ignore the partition as if you have a 15TB drive instead, so you can continue using TM for backups but have a way back to an exact copy of your Mac boot drive as it is right now.
With a bootable clone, you can choose that boot drive instead of the one built in to boot into it exactly as you cloned it. For example, I have this for macOS 10.13.6 to hang on to certain software functionality that wouldn't function in 10.14 and newer. I also have an old Snow Leopard backup to be able to use a few pieces of software that need Rosetta 1. I have 1 Mac that can boot into either. So on startup, I can let it boot into the newest macOS as default or override to boot into 10.13.6 or even Snow Leopard. If the latter, it is as if it is a Mac from years ago, with all the files/folders/apps as they were- and worked- on those earlier versions of macOS and OS X.
There are some who talk about this rule of not upgrading macOS until the .3 or .4 update. This approach allows you to take shot at the .0 update when released. If it turns out to be buggy/messy/problematic, you can always go back as long as you have a cloned boot drive with what you consider the last stable release (for your needs). TM backups aren't backing up everything, so it is not the same.
If you've been at this long, you probably have some old, smaller hard drives laying around doing nothing. Boot drives in Macs are usually not very big (would even 1TB be "average"?). You can probably put an old, maybe "retired" (because it is "too small") drive back to work. Else, buying an external to match whatever size you have in your Mac is probably an external that doesn't cost more than $150 at most... and more likely less than $100.
