Good comment by ZD.
Regarding the deleted 8.1 partition, if in fact that existed as a partition that was separate from the W10 partition, as opposed to having been in a partition that was overwritten by the W10 installation, you might be able to recover it by using the partition recovery function
that is provided in the free version of the MiniTool Partition Wizard
. I haven't had occasion to use the partition recovery function of that program, but I've used some of its other partition management functions, and it is a remarkably good program, especially considering that it's free.
I'd assume that it can be installed into W10, given its advertised compatibility with every previous version of Windows back to W2000.
Also, FWIW, I'm a big believer in disk imaging. If you had an image of each of the partitions on the drive, or of the entire drive, stored on a separate drive, getting back to where you were would be very easy. I use Terabyte Unlimited's Image For Windows
. I create images of the "c" drive on each of my computers about once a month, and store the two most recent image files on a separate drive. If a drive were to fail, become corrupted, or have some hard to fix software issue develop, rather than having to reinstall and set up Windows, programs, drivers, updates, etc. from scratch I could very quickly just restore the most recent image, and replace the restored data files with backups which I do daily.
Good luck. Best regards,