There has been some confusion around this particular subject, so I'll confirm the answer. You can use Mavericks in 2021! Here is what you'll need to do for each aspect.
Web Browser
You can simply use Firefox Extended Support Release. However, that doesn't work so well with Teams. Google Chrome is still available for Mavericks, in fact it is available for anything 10.6 up! So that's easy to resolve.
Microsoft Office
You can still get Microsoft Office 2011 for Mavericks. You might still have the Install DVD, or you can download it online and possibly get a licence (I haven't looked into that, as I use my school's subscription). You could also use Apple's 'iWork' suite (see below).
iWork
iWork users, you are in luck. You can download iWork '09 trial DMG, and then upgrade, or you can, if you purchased iWork before, you can just check purchases in App Store.
Anything else?
Most apps will have versions that will work on Mavericks. If there's something on the App Store that you want, follow these steps. Check in your purchases and see if it is there. If it is, click Install. If the app is available for mavericks, it will either start downloading, or it will ask you to confirm the download of a previous version of the app. If not, you will get a flat-out refusal to download the app.
If you want, I can possibly check for other software and see if it is compatible. I have catalogued most software I use.
How to even get Mavericks
I did skip the most important part: How to get Mavericks. That's easy. Just go to allmacworld.com and search for 'Mavericks' and select 10.9.5. Do not select 10.9 or 10.9.0. That download is for Hackintoshes.
I hope this information is helpful.
mac23456
Web Browser
You can simply use Firefox Extended Support Release. However, that doesn't work so well with Teams. Google Chrome is still available for Mavericks, in fact it is available for anything 10.6 up! So that's easy to resolve.
Microsoft Office
You can still get Microsoft Office 2011 for Mavericks. You might still have the Install DVD, or you can download it online and possibly get a licence (I haven't looked into that, as I use my school's subscription). You could also use Apple's 'iWork' suite (see below).
iWork
iWork users, you are in luck. You can download iWork '09 trial DMG, and then upgrade, or you can, if you purchased iWork before, you can just check purchases in App Store.
Anything else?
Most apps will have versions that will work on Mavericks. If there's something on the App Store that you want, follow these steps. Check in your purchases and see if it is there. If it is, click Install. If the app is available for mavericks, it will either start downloading, or it will ask you to confirm the download of a previous version of the app. If not, you will get a flat-out refusal to download the app.
If you want, I can possibly check for other software and see if it is compatible. I have catalogued most software I use.
How to even get Mavericks
I did skip the most important part: How to get Mavericks. That's easy. Just go to allmacworld.com and search for 'Mavericks' and select 10.9.5. Do not select 10.9 or 10.9.0. That download is for Hackintoshes.
I hope this information is helpful.
mac23456