So I emailed Aldi and they got back to me already, good and bad news, they're not going forever:
Thank you for contacting Aldi Customer Services regarding the availability of the chicken burger.
I have checked the availability for the spicy vegan chicken burger and must advise you that this product is a seasonal product and only available in stores during the winter season.
Please accept our apologies for any disappointment caused.
Once again, thank you for contacting Aldi Customer Services and we look forward to your continued custom.
Customer Service Advisor
ALDI Stores Ltd