create a frontend list of all active CBsubs users?

Yes, if the key doesn't match the URL access will be denied.

why would my key be different now than a few years back when I installed CBsubs?

How would I include this "curr_yr" variable into this query like below?

$curr_yr = date('Y') ;

$query = "SELECT COUNT(`id`)
from j17_cbsubs_subscriptions
WHERE id!=62
and id !=299
and expiry_date < '"$curr_yr"-12-31 23:59:58'
AND status='A'";

why would my key be different now than a few years back when I installed CBsubs?

I've no idea. I guess the code for generating it has changed or if it's using Joomla secret key then it'd be different per Joomla install.

How would I include this "curr_yr" variable into this query like below?

With the SQL function YEAR(). See the below MYSQL documentation for date function usage.

dev.mysql.com/doc/refman/5.6/en/date-and-time-functions.html