It is unfortunate indeed. Most companies would ensure that impact on clients is minimised and they are well informed. You have all the information you need to identify you user base. There has been a lack of engagement of those clients that are impacted. Such changes come as a shock. This approach seem very cavalier.
It is not just an inconvenience but also a shame that IG has chosen to forego clients whose needs you used to cater for. Less profits from this user base!
Please consider altering the website, so that, like me other users do not falsely think Windows mobile users can continue using the app.