Deena's post only mentioned "of at least one large RCT underway, with results expected in a few years" without further reference, but on cursory googling it might be the CRADLE trial?

The Cement-based flooRs AnD chiLd hEalth trial is an individually randomised trial in Sirajganj and Tangail districts, Bangladesh. Households with a pregnant woman, a soil floor, walls that are not made of mud and no plan to relocate for 3 years will be eligible. We will randomise 800 households to intervention or control (1:1) within geographical blocks of 10 households to account for strong geographical clustering of enteric infection. Laboratory staff and data analysts will be blinded; participants will be unblinded. We will instal concrete floors when the birth cohort is in utero and measure outcomes at child ages 3, 6, 12, 18 and 24 months. 

The primary outcome is prevalence of any STH infection (Ascaris lumbricoides, Necator americanus or Trichuris trichiura) detected by quantitative PCR at 6, 12, 18 or 24 months follow-up in the birth cohort. Secondary outcomes include household floor and child hand contamination with Escherichia coli, extended-spectrum beta-lactamase producing E. coli and STH DNA; child diarrhoea, growth and cognitive development; and maternal stress and depression.

We will report findings on ClinicalTrials.gov, in peer-reviewed publications and in stakeholder workshops in Bangladesh.

While GiveWell doesn't seem to have looked into this specifically, this 2015 review of GiveDirectly mentioned that lack of cement floors was in one of GiveDirectly's two sets of eligibility criteria for its standard campaigns:

Thatched roofs: To date, GiveDirectly has used housing materials to select recipients in all of its standard campaigns, enrolling households who live in a house made of organic materials (thatched roof, mud walls, and a mud floor) and excluding households with iron roofs, cement walls, or cement floors.170 In GiveDirectly's campaigns in Kenya, about 35-45% of households have been eligible based on these criteria, while in Uganda about 80% of households have been found to be eligible.171 ... 

Happier Lives Institute's 2021 annual review did mention cement flooring among the "micro-interventions" they wanted to look into (alongside deworming, cataract surgery, digital mental health interventions, etc), but I haven't seen anything by them since on this, so I assume it didn't pass their internal review for further analysis. 

I have the opposite intuition for biodiversity. People have been studying ecosystem services for decades and higher biodiversity is associated with increased ecosystem services, such as clean water, air purification, and waste management. Higher biodiversity is also associated with reduce transmission of infectious diseases by creating more complex ecosystems limiting pathogen spread. Then we have the actual and possible discovery of medicinal compounds and links with biodiversity and mental health. These are high level examples of the benefits. The linked article gives the possibility of impact by considering two effects from bats and vultures. Multiply that effect by 1000+ other species, include all the other impacts previously mentioned and I can see how this could be high impact. 

I'm not seeing where Deena wrote that biodiversity in general was important? 

Both studies suggest that protecting certain animal populations might have large, direct effects on human health that we’re overlooking. But there are good reasons to be cautious. These are outlier results; there isn’t much else in the way of evidence for estimates of this magnitude for the impact of biodiversity loss on human mortality. There’s also the possibility of publication bias. In particular, since both papers come from the same author, this may be driven by a file drawer effect, where a researcher looks at many potential similar cases but the null findings are less likely to see the light of day.

Still, if these effects are real, they could change how we think about conservation. Saving vultures or bats wouldn’t just be about biodiversity—it could also be a form of public health policy.