Anti-virus programs: Yosemite & Snow Leopord on Macbook Pro mid-2009

None needed.

Since you have decided to run an older version of macOS, you are leaving yourself open to some additional hazard which have been fixed on the newer version of macOS.

Having said that, I still strongly suggest against the installation of any anti-virus software. All of those packages only cause problems and offer no additional security.

What might the name of these expensive software packages be?

Are there any plans to upgrade?

+1 for none needed.

I would uninstall Bitdefender. It tends to interfere with the computer's operation while providing minimal to no benefit unless you work with Windows files. Most long time posters recommend not using antivirus software or cleaning software such as CleanMyMac. They can adversely affect normal computer operation.

Bitdefender uninstall

Bitdefender uninstall (2)

I ran an early 2008 iMac using El Capitan and had no problems. While the decision to upgrade is yours, based on my experience, there is no reason not to upgrade unless there is a software conflict as Limnos mentioned.

El Capitan

Check to make sure your applications are compatible.

Application Compatibility

El Capitan 10.11 Compatibility information

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

I use an older Mac and frankly the last time I think I saw something nasty on a Mac computer was maybe 20 years ago when running OS 8. That was the sole virus I have seen in 3 decades of computer use. Nobody's really bothering writing true viruses for Macs. Maybe some malware but I get the impression you have to pretty much invite it to come in and install itself and a modicum of cautious behavior goes a long way. I periodically scan using Malwarebytes (the free version) but it has yet to detect anything. If you're already running an older, slow computer you don't want to install any scanning software or you'll tie the whole thing down.

If you're concerned about security, these old versions of macOS are definitely a consideration.

You're running versions of OS X with very old TLS support for secure network connections, and attempts to access various web sites and secure services will be blocked by remote servers configured to modern security. OS X releases prior to OS X 10.9 don't have secure TLS support.

The older versions are also missing security-related features such as System Integrity Protection.

Another and more general security-related consideration is backups. Yes, backups are part of security. Both continuous backups via Time Machine or CCC or otherwise, as well as the occasional or periodic backups that are disconnected from your systems and preferably also rotated off-site. Or some other appropriate permutation. Backups are the path to recovery from loss, theft, damage, malware, operator error, and from failed hardware. And the hardware of Mac systems that can run Snow Leopard OS X 10.6 is getting old.

It's possible to run older releases of OS X as guests in newer Mac systems, using a virtual machine. That's an approach that would allow you to move forward while maintaining access to some of those older apps, and well worth testing. That all assumes this Mac can support newer releases of macOS.

A 2009 MacBook Pro is limited to upgrading as far as El Capitan if I remember correctly. I don't know if there is any real reason why not as long as you aren't limited by a need to run legacy software. The main reasons for not upgrading are if you are running your MBP in its original configuration in which case you probably have a slower hard drive and low RAM. El Capitan was programmed during an era when most new Macs had SSD drives which are faster, and although Apple always says in its specs you can run its systems with really low RAM, sure, if all you do is run TextEdit. 😝 Most people go for higher, and at least 8 GB these days. I just helped somebody upgrade their 2009 MBP hardware so it can run El Capitan with 8GB RAM and a SSD.