You can put a ‘no hard drive’ label on the computer if you remove the drive. That will let regular people know they probably don’t want it. Lack of a drive is no problem for the old hardware folk because they probably have a closet full of them already. It best to leave any drive holder with the mac though (taped inside so it doesn’t rattle is best); they’re more of a pain to replace especially for IDE and scsi era drives.
Macs can do proper secure format with no problem in Disk Utility, at least as far back as Disk Utility existed. You need to erase the disk, not the volume. Select the ‘Seagate blah’ line in Disk Utility, not the indented ‘Macintosh HD’ line. (On recent systems, you’ll have to be in ‘Show All Devices’ view to do this.) Click on Erase, and there’s a Security Options… button. Select your desired level of wiping. More rewrites of random data will take longer, but for a small old disk that may not be too bad.
In the future, when Macs with T chips start getting old enough to pass on, if possible you should boot into Recovery and at least enable booting from external drives and probably turn off Secure Boot. Otherwise it may not be useable as a computer in short order. Since the T chips hardware encrypt the internal SSD, a simple erase of the data volume while in Recovery should be enough.
[Actually. you should enable external booting for your new hardware, too, unless you think you might be targeted. Being able to boot from an external drive is often the only way to get a computer working again if there’s a problem.]