Is it okay to use openwrt 19.10 now?
because i have a 8mb flash device 
using it in pppoe connection from my isp
No, 19.07 has been EOL and unsupported for many years now. It has many security vulnerabilities that will never be patched.
8MB of flash is sufficient to run even the latest (23.05), provided that you have at least 64MB of RAM.
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If i use openwrt 23 on archer c60( 8 mb flash), total available memory left is 4-5% and router suddenly hangs. 
Openwrt 19.10 gives butter smooth performance.
You can use whatever version you want, but it is not recommended nor safe to use 19.07 at this time.
You'd be best suited by creating a new thread to troubleshoot the situation with 23.05.
i have seen OpenWrt 21.02.7 released this may, 2023. is it recommended to use? or is it also unsafe like openwrt 19.10?
21.02 is also eol and unsupported.
22.03.5 or 23.05.0 would be the recommended options.
If I were you I would soon buy new hardware with at least 64 MiB flash and 256 MiB main memory, before 22.03 will be end of life in about 2024.
Cheaper router models could for example use the MediaTek MT7621 SoC: https://openwrt.org/tag/mt7621?do=showtag&tag=mt7621
Recent popular high performance models have 802.11ax WiFi-6 support:
https://openwrt.org/tag/802.11ax?do=showtag&tag=802.11ax
Older devices with up to and including 8 MiB flash and up to and including 64 MiB memory will be soon at their end of life: https://openwrt.org/supported_devices/openwrt_on_864_devices
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This thread should be closed and noted that this version is not supported anymore. Users should be suggested to use newer versions. We should not revive thread, which is one year old.
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Agree. Closing this now.
19.07 is EOL and obsolete. It is unsupported and there are no patches available since its release in April 2022.
Signal boosting the statement at the beginning of this thread:
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