Haven't updated since version 2.6.2949 - are there any potential negative consequences of not updating? If so, what are they? Thanks.
No software is perfect. Like any other well-managed software project, Craft CMS is regularly updated with new features, improvements and bug fixes.
Occasionally, the changes will relate to a specific security issue that has been discovered in the software.
By not updating, you are missing out on the new features, improvements and bug fixes that make Craft CMS better.
If you don't update when a new security issue is patched, then you are leaving your site at risk of exploitation of the security vulnerability. Depending on the nature of the specific vulnerability, that could mean a threat to your site, your business, the web server that your site is running on or all of them.
Note that this applies to all software, not just Craft CMS.
The Craft CMS changelog is a good resource to see what changes are included in each update.