Hmm... maybe Undertale actually made a reference to this? Idk.
The earliest date I found this image with is 2016, whereas Undertale came out in 2015 (ikr, how could it have been 3 years already?) so I think this sign is a reference to Undertale. But I could be wrong, google searches usually show the most relevant thing so it showed me this image from 2016 and some reposts from 2017 and 2018, but it could be older and just lost within the depths of the internet.
The earliest date I found this image with is 2016, whereas Undertale came out in 2015 (ikr, how could it have been 3 years already?) so I think this sign is a reference to Undertale. But I could be wrong, google searches usually show the most relevant thing so it showed me this image from 2016 and some reposts from 2017 and 2018, but it could be older and just lost within the depths of the internet.