Yes the penalty expires in 2019, but it is still in effect for 2018. So, if you file taxes in 2019 for 2018 and for every month (over) 3 months not insured, will be penalized on your 2018 taxes.
Now the problem with employer insurance is that you can't cancel till the (next) open enrollment. If you had a private policy then you could buy for 2 months and cancel in January, since with (private) insurance can only buy at open enrollment in Nov/Dec to be effective Jan/Feb of the (next) year..
Now many employers like private have open enrollment in January, but many do not. I do remember years ago (retired) that I had some employers that renewed in June or September, thus is the open enrollment is in June or September, then only then you can cancel.
So it is still possible if you worked this year and if you made enough to file taxes and have been uninsured for 10 months of 2018, then you will be penalized for 9 months on your 2018 taxes. Or, talk to HR and find out when the employer has open enrollment to find out (when) you can opt out.
Yes, health insurance is pricey, but you like many young people think they will never need it (till) you have a car accident or break your arm/leg playing sports as an example.