It is hoped that 2.5Gbps support can be added to the SFP + of rb5009.
It has been checked that his switching chip supports 2.5Gbps. I hope to support it.
https://www.marvell.com/content/dam/marvell/en/public-collateral/switching/marvell-switching-link-street-88e6393x-product-brief-2019-01.pdf
Support for what SFP+ module?
He is referring to HGSMII such as in GPON SFPs has been added to some switches since v7.3beta33
*) sfp - added 2.5Gbps rate for SFP+ and QSFP+ interfaces on 98DXxxxx and 98PX1012 switches (requires disabled auto-negotiation);
BINGO!
+1
My rb5009 works with GPON SFP module in 1Gbps mode. The GPON SFP module supports 2.5Gbps, but rb5009 not.
I Hope Mikrotik add support on RB5009 for 2500BASE-X on SFP+ Port soon
Have same problem with my ODI DFP-34X-2C2 gpon onu can only working on 1Gbps.
A lot of GPON SFPs support 2.5Gbps(HiSGMII and 2500baseX) and the Marvell switch chip in the 5009 series also supports this mode.
Only thing missing is Mikrotik firmware support.
Please Mikrotik add support for this feature.
Yes please, on all CCR, CRS and RBs 
What’s new in 7.7beta4 (2022-Oct-27 09:00):
*) sfp - added 2.5G SFP module support for RB5009;
Seems it is supported now in latest beta can you check.
cat /proc/lan_sds/lan_sds_cfg
lan_sds_mode = 4(HiSGMII PHY)
flash get LAN_SDS_MODE
LAN_SDS_MODE=0
I can confirm it is working
By turning auto-neg off and setting it on 2.5Gbps Full-Duplex
For those with the RB5009 and a GPON SFP, Does Forcing 2.5Gbps rate work on v7.9rc1?
Well it does not (!) work with the 1gbps sfp adapters from fs.com 
Why would you expect a 1Gbps SFP to work if you attempt to send 2.5Gbps through it.