Requirement:
Aruba central should be running in 2.5.x.
Solution:How to disable MU/High efficiency for the SSID in Aruba central for IAP.
In Aruba central code 2.5.x, in order to disable MU/High efficiency please go to the SSID advanced settings > Bandwidth control > disabled AC and AX.
Configuration: 
VerificationWith HE enabled
Aruba AP BSS Table
------------------
bss ess port ip phy type ch/EIRP/max-EIRP cur-cl ap name in-t(s) tot-t flags
--- --- ---- -- --- ---- ---------------- ------ ------- ------- ----- -----
20:a6:cd:xx:xx:xx Test-IAP ?/? 169.254.183.48 g-HE ap 11/19.9/19.9 3 20:4c:03:xx:xx:xxTest-303H 0 6h:45m:2s
wlan ssid-profile Test-IAP
enable
index 0
type employee
essid Test-IAP
utf8
wpa-passphrase 2458bcf1b160f20ff9bc2c2240412614f654bdf6a9173b38
opmode wpa2-psk-aes
max-authentication-failures 0
vlan 10
auth-server InternalServer
rf-band 2.4
captive-portal disable
dtim-period 1
broadcast-filter all
dmo-channel-utilization-threshold 90
local-probe-req-thresh 0
max-clients-threshold 64
With HE disabled.
Aruba AP BSS Table
------------------
bss ess port ip phy type ch/EIRP/max-EIRP cur-cl ap name in-t(s) tot-t flags
--- --- ---- -- --- ---- ---------------- ------ ------- ------- ----- -----
20:a6:cd:xx:xx:xx Test-IAP ?/? 169.254.183.48 g-HT ap 11/19.9/19.9 3 20:4c:03:xx:xx:xx-Test-303H 0 6h:45m:2s
wlan ssid-profile ShweAlex-IAP
enable
index 0
type employee
essid Test-IAP
utf8
wpa-passphrase db74991876281e48fd5799fdf63e0d8ead9eafce68351415
opmode wpa2-psk-aes
max-authentication-failures 0
vlan 10
auth-server InternalServer
rf-band 2.4
captive-portal disable
dtim-period 1
broadcast-filter all
dmo-channel-utilization-threshold 90
local-probe-req-thresh 0
max-clients-threshold 64
very-high-throughput-disable
high-efficiency-disable
In the running configuration, you can see “high-efficiency-disable” got pushed post making changes in Bandwidth control.