In the 126.96.36.199 and 120 builds of Chinese variant CLT_AL00 P20 Pro emui 9 beta, as well as all emui builds, beta or not, containing the wechat fingerprint payment patch, Huawei seemed to have pulled a oppo and removed the option to change the default launcher. Would not be surprising considering they just removed the option to unlock bootloader.
*update: launching default app settings from activity launcher appear to show the launcher options again however it is still impossible to select one as default as the buttons are greyed out.
*update: further investigation shows that every chinese build of emui in the past month that include support for wechat fingerprint payment, both emui 8.2 and emui 9 and across both hauwei and honor, seem to have removed the ability to use third party launchers, the resaosn I did not see this before, as it seems, is that Huawei club has been diligent in deleting the threads up until now. It is incredibly sad for the active community of Huawei users, me included, who want a little more out of their phones, I hope this issue can be give more public attention amid coverage of the mate 20 launch so people know what they are getting into.
***UPDATE ADB UNINSTALL AND DISABLING THE HUAWEI HOME LAUNCHER NO LONGER WORKS
****UPDATE EMUI 10 BETA STILL HAS NOT ADDRESSED THE ISSUE IN CHINA ROMS